U.S. patent application number 13/223081 was filed with the patent office on 2013-02-28 for carbon dioxide-resistant portland based cement composition.
This patent application is currently assigned to Halliburton Energy Services, Inc.. The applicant listed for this patent is David Kulakofsky, Ashok K. Santra. Invention is credited to David Kulakofsky, Ashok K. Santra.
Application Number | 20130048284 13/223081 |
Document ID | / |
Family ID | 46682959 |
Filed Date | 2013-02-28 |
United States Patent
Application |
20130048284 |
Kind Code |
A1 |
Santra; Ashok K. ; et
al. |
February 28, 2013 |
CARBON DIOXIDE-RESISTANT PORTLAND BASED CEMENT COMPOSITION
Abstract
The invention provides a carbon dioxide-resistant hydraulic
cement composition. The inventive composition comprises a Portland
cement, Class C fly ash and water. The Class C fly ash is present
in the composition in an amount in the range of from about 5% to
less than about 30% by weight based on the total weight of the
cementitious components in the composition. In another aspect, the
invention provides a method of cementing in a carbon dioxide
environment. In yet another aspect, the invention provides a method
of enhancing the recovery of a hydrocarbon fluid from a
subterranean formation.
Inventors: |
Santra; Ashok K.; (Norman,
OK) ; Kulakofsky; David; (Katy, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Santra; Ashok K.
Kulakofsky; David |
Norman
Katy |
OK
TX |
US
US |
|
|
Assignee: |
Halliburton Energy Services,
Inc.
Houston
TX
|
Family ID: |
46682959 |
Appl. No.: |
13/223081 |
Filed: |
August 31, 2011 |
Current U.S.
Class: |
166/292 ;
106/709 |
Current CPC
Class: |
C04B 2103/0035 20130101;
C04B 2103/0036 20130101; C04B 2103/46 20130101; C09K 8/594
20130101; C04B 2103/50 20130101; C09K 8/467 20130101; C04B 7/02
20130101; C04B 2103/50 20130101; C04B 28/021 20130101; C04B 28/04
20130101; C04B 28/021 20130101; C04B 7/02 20130101; C04B 2103/46
20130101 |
Class at
Publication: |
166/292 ;
106/709 |
International
Class: |
E21B 33/13 20060101
E21B033/13; C04B 18/06 20060101 C04B018/06 |
Claims
1. A carbon dioxide-resistant hydraulic cement composition,
comprising: a Portland cement; Class C fly ash present in an amount
in the range of from about 5% to less than about 30% by weight
based on the total weight of the cementitious components in said
composition; and water present in an amount sufficient to form a
slurry.
2. The carbon dioxide-resistant hydraulic cement composition of
claim 1, wherein said Portland cement is selected from API Class G
type Portland cement and API Class H type Portland cement.
3. The carbon dioxide-resistant hydraulic cement composition of
claim 2, wherein said Portland cement is API Class H type Portland
cement.
4. The carbon dioxide-resistant hydraulic cement composition of
claim 1, further comprising a fluid loss additive.
5. The carbon dioxide-resistant hydraulic cement Composition of
claim 1, further comprising a defoamer.
6. The carbon dioxide-resistant hydraulic cement composition of
claim 1, wherein said Class C fly ash is present in said
composition in an amount in the range of from about 15% to about
28% by weight based on the total weight of the cementitious
components in said composition.
7. The carbon dioxide-resistant hydraulic cement composition of
claim 6, wherein said Class C fly ash is present in said
composition in an amount of about 25% by weight based on the total
weight of the cementitious components in said composition.
8. The carbon dioxide-resistant hydraulic cement composition of
claim 1, wherein said Class C fly ash is present in said
composition in an amount of from about 5% to less than about 30% by
weight based on the total weight of said Portland cement and Class
C fly ash in said composition.
9. The carbon dioxide-resistant hydraulic cement composition of
claim 1, wherein said composition has a cement density in the range
of from about 12 to about 19 pounds of cementitious components per
gallon of water in said slurry.
10. A method of cementing in a carbon dioxide environment,
comprising: preparing a carbon dioxide-resistant hydraulic cement
composition, said carbon dioxide-resistant hydraulic cement
composition including: a Portland cement; Class C fly ash present
in an amount in a range of from about 5% to less than about 30% by
weight based on the total weight the cementitious components in
said composition; and water present in an amount sufficient to form
a slurry; placing said carbon dioxide-resistant hydraulic cement
composition in said carbon dioxide environment; and allowing said
carbon dioxide-resistant hydraulic cement composition to set.
11. The method of claim 10, wherein said Portland cement of said
carbon dioxide-resistant hydraulic cement composition is selected
from API Class G type Portland cement and API Class H type Portland
cement.
12. The method of claim 11, wherein said Portland cement of said
carbon dioxide-resistant hydraulic cement composition is API Class
H type Portland cement.
13. The method of claim 10, wherein said Class C fly ash is present
in said composition in an amount in the range of from about 15% to
about 28% by weight based on the total weight the cementitious
components in said composition.
14. The method of claim 10, wherein said Class C fly ash is present
in said composition in an amount of from about 5% to less than
about 30% by weight based on the total weight of said Portland
cement and Class C fly ash in said composition.
15. A method of enhancing the recovery of a hydrocarbon from a
subterranean formation, comprising: placing one or more injection
wells into said subterranean formation, said injection well
including a casing cemented into place using a hydraulic cement
composition; placing one or more production wells into said
subterranean formation, said production well(s) including a casing
cemented into place using a hydraulic cement composition; injecting
a flooding composition including carbon dioxide and water through
one or more of said injection wells into said subterranean
formation in order to pressurize said subterranean formation and
drive said hydrocarbon toward said production well(s), wherein said
hydraulic cement composition utilized to cement the casing into
place in at least one of said injection well(s) and said production
well(s) is a carbon dioxide corrosion-resistant hydraulic cement
composition that includes: a Portland cement; Class C fly ash
present in an amount in a range of from about 5% to less than about
30% by weight based on the total weight the cementitious components
in said composition; and water present in an amount sufficient to
form a slurry.
16. The method of claim 15, wherein said Portland cement of said
carbon dioxide-resistant hydraulic cement composition is selected
from API Class G type Portland cement and API Class H type Portland
cement.
17. The method of claim 16, wherein said Portland cement of said
carbon dioxide-resistant hydraulic cement composition is API Class
H type Portland cement.
18. The method of claim 15, wherein said carbon dioxide-resistant
hydraulic cement composition further comprises a fluid loss
additive.
19. The method of claim 15, wherein said Class C fly ash is present
in said composition in an amount in the range of from about 15% to
about 28% by weight based on the total weight the cementitious
components in said composition.
20. The method of claim 19, wherein said Class C fly ash is present
in said composition in an amount of from about 5% to less than
about 30% by weight based on the total weight of said Portland
cement and Class C fly ash in said composition.
Description
BACKGROUND OF THE INVENTION
[0001] Hydraulic cement compositions are often used in applications
that are or will be associated with a relatively high level of
carbon dioxide. For example, hydraulic cement compositions are used
to encase the well bores of injection and production wells used in
connection with enhanced oil recovery techniques. A fluid commonly
used to flood the formation in such techniques is carbon dioxide.
Specifically, the carbon dioxide is injected into the formation
together with water through one or more injection wells to drive
hydrocarbons in the formation toward one or more production wells.
This technique has proved to be effective in increasing production
of the hydrocarbons from the formation.
[0002] Hydraulic cement compositions are also used in other
applications that involve or may involve a carbon dioxide
environment. Examples include formation sealing applications and
other cementing applications associated with oil, gas, water and
geothermal wells and carbon capsule storage applications associated
with power plants.
[0003] A problem that can result from the use of hydraulic cement
compositions in applications that are or will be associated with a
carbon dioxide environment is corrosion of the hydraulic cement by
carbonic acid and other corrosive compounds formed by reactions
between the carbon dioxide, water and potentially other compounds
in the environment. Carbonic acid and other corrosive compounds
formed from carbon dioxide can react with and penetrate into
hardened hydraulic cement thereby lowering the compressive strength
thereof. For example, carbonic acid corrosion can cause the
production casing of an oil and gas well to fail resulting in
undesired migration of fluids between the formation and well bore
and other serious problems. Similar problems and adverse
consequences can occur in other applications in which hydraulic
cement compositions are used in carbon dioxide environments.
[0004] There is a need for a hydraulic cement composition that is
resistant to corrosion by carbonic acid in downhole and other
environments and that can be used in effective and efficient
manners.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a schematic cross-sectional view illustrating one
embodiment of the inventive method of enhancing the recovery of a
hydrocarbon from a subterranean formation.
[0006] FIG. 2 is a graph corresponding to Example 1.
BRIEF SUMMARY OF THE INVENTION
[0007] In one aspect, the invention provides a carbon
dioxide-resistant hydraulic cement composition. The inventive
carbon dioxide-resistant hydraulic cement composition comprises a
Portland cement, Class C fly ash and water. The Class C fly ash is
present in the composition in an amount in the range of from about
5% to less than about 30% by weight based on the total weight of
the cementitious components in the composition. The water is
present in the composition in an amount sufficient to form a
slurry.
[0008] In another aspect, the invention provides a method of
cementing in a carbon dioxide environment. The inventive method of
cementing in a carbon dioxide environment comprises the steps of
preparing a carbon dioxide-resistant hydraulic cement composition,
placing the carbon dioxide-resistant hydraulic cement composition
in the carbon dioxide environment, and allowing the carbon
dioxide-resistant hydraulic cement composition to set.
[0009] The carbon dioxide-resistant hydraulic cement composition
utilized in the inventive method of cementing in a carbon dioxide
environment includes a Portland cement, Class C fly ash, and water.
The Class C fly ash is present in the composition in an amount in
the range of from about 5% to less than about 30% by weight based
on the total weight of the cementitious components in the
composition. The water is present in the composition in an amount
sufficient to form a slurry.
[0010] In yet another aspect, the invention provides a method of
enhancing the recovery of a hydrocarbon from a subterranean
formation. The inventive method of enhancing the recovery of a
hydrocarbon from a subterranean formation comprises the steps of:
(a) placing one or more injection wells into the subterranean
formation, the injection well(s) including a casing cemented into
place using a hydraulic cement composition; (b) placing one or more
production wells into the subterranean formation, the production
well(s) including a casing cemented into place using a hydraulic
cement composition; and (c) injecting a flooding composition
including carbon dioxide and water through one or more of the
injection wells into the subterranean formation in order to
pressurize the subterranean formation and drive the hydrocarbon
toward the production well(s).
[0011] The hydraulic cement composition utilized to cement the
casing into place in at least one of the injection well(s) and
production well(s) is a carbon dioxide corrosion-resistant
hydraulic cement composition. The carbon dioxide
corrosion-resistant hydraulic cement composition includes a
Portland cement, Class C fly ash and water. The Class C fly ash is
present in the composition in an amount in the range of from about
5% to less than about 30% by weight based on the total weight of
the cementitious components of the composition. The water is
present in the composition in an amount sufficient to form a
slurry.
DETAILED DESCRIPTION
[0012] The invention includes a carbon dioxide-resistant hydraulic
cement composition, a method of cementing in a carbon dioxide
environment (the "inventive cementing method"), and a method of
enhancing the recovery of a hydrocarbon fluid from a subterranean
formation (the "inventive recovery enhancing method"). The
inventive carbon dioxide-resistant hydraulic cement composition is
utilized in both the inventive cementing method and the inventive
recovery enhancing method.
[0013] As used herein and in the appended claims, a carbon dioxide
environment means an environment that contains or may contain an
amount of carbon dioxide capable of causing corrosion to hydraulic
cement. For example, a carbon dioxide environment can be an
environment that already includes carbon dioxide and water (for
example, a subterranean area that contains carbon dioxide and
water, and in which a carbon storage capsule is placed) or an
environment that may be subjected to or otherwise include carbon
dioxide and water in the future (for example, a subterranean
formation in which carbon dioxide and water are subsequently
injected in connection with an enhanced oil recovery
operation).
[0014] The inventive carbon dioxide-resistant hydraulic cement
composition comprises a Portland cement, Class C fly ash and water.
The water is present in the composition in an amount sufficient to
form a slurry.
[0015] The Portland cement utilized in the inventive composition is
preferably selected from Class G type Portland cement and Class H
type Portland cement as classified according to API Specification
for Materials and Testing (API Specification 10A), published by The
American Petroleum Institute (hereinafter "API Class G (or API
Class H) type Portland cement"). More preferably, the Portland
cement utilized in the inventive composition is API Class H type
Portland cement.
[0016] Fly ash is very fine ash produced by the combustion of
powdered coal. The Class C fly ash utilized in the inventive
composition is Class C fly ash as defined in ASTM Specification C
618. Class C fly ash has pozzolanic and cementitious properties.
The Class C fly ash is present in the composition in an amount in
the range of from about 5% to less than about 30% by weight,
preferably in the range of from about 15% to about 28% by weight,
and most preferably about 25% by weight, based on the total weight
of the cementitious components in the composition. As used herein
and in the appended claims, a cementitious component is a component
that has the properties of hydraulic cement in that it chemically
combines with other ingredients to form a hydrated cement. As used
herein and in the appended claims, the expressed percents by weight
of the Class C fly ash and Portland cement are based on a dry
weight basis.
[0017] In one embodiment, the cementitious components utilized in
the inventive composition are Portland cement and Class C fly ash.
Accordingly, in this embodiment, the Class C fly ash is present in
the composition in an amount in the range of from about 5% to less
than about 30% by weight, preferably in the range of from about 15%
to about 28% by weight, and most preferably about 25% by weight,
based on the total weight of the Portland cement and the Class C
fly ash present in the composition.
[0018] Additional components can optionally be included in the
inventive composition depending on the application. For example, in
one embodiment, a fluid loss additive is included in the
composition. An example of a suitable fluid loss additive is
Halad.RTM.-344, a fluid loss additive marketed by Halliburton
Energy Services, Inc. and comprising a random copolymer of
2-acrylamide-2-propane sulfonic acid and N,N-dimethyl acrylamide. A
defoamer can also be used in the inventive composition. An example
of a suitable de-foaming agent is D-AIR 3000L.TM., a defoamer
marketed by Halliburton Energy Services, Inc. and comprising an
internal olefin (C.sub.14-C.sub.18), an alkaline hydrophobic
precipitated silica, and polypropylene glycol 4000. Other
components that can be utilized in the inventive composition
include retarding agents, accelerating agents, silica, elastomers,
fibers, hollow beads and foaming agents. The particular additives
and the amount of such additives utilized will depend on the
particular application.
[0019] The components of the inventive composition are admixed
together to form a pumpable slurry. The density of the slurry can
vary depending on the application. Generally, the density of the
slurry is in the range of from about 12 to about 19 pounds per
gallon of water in the slurry. The slurry ultimately hardens and
sets into a carbon dioxide corrosion-resistant hydraulic
cement.
[0020] The inventive cementing method comprises the steps of
preparing a carbon dioxide-resistant hydraulic cement composition,
placing the carbon dioxide-resistant hydraulic cement composition
in the carbon dioxide environment, and allowing the carbon
dioxide-resistant hydraulic cement composition to set. The carbon
dioxide-resistant hydraulic cement composition utilized in the
inventive cementing method is the inventive carbon
dioxide-resistant hydraulic cement composition.
[0021] The inventive cementing method can be used in connection any
cementing application involving a carbon dioxide environment.
Examples include cementing applications involving wells (for
example, oil, gas, water, and geothermal wells) penetrating
subterranean formations, including primary cementing applications,
formations sealing and consolidation applications, formation of
cement plugs for various purposes, and remedial cementing
applications. Other cementing applications involving a carbon
dioxide environment and in which the inventive cementing method can
be utilized include the formation of underground cement capsules
for storing carbon from power plants and cementing applications
used in connection with in situ combustion techniques used in
connection with coal gasification.
[0022] The inventive recovery enhancing method can be utilized to
enhance the production of a hydrocarbon (such as crude oil and/or
natural gas) from partially depleted reservoirs thereof. The
inventive recovery enhancing method comprises the steps of: (a)
placing one or more injection wells into the subterranean
formation, the injection well(s) including a casing cemented into
place using a hydraulic cement composition; (b) placing one or more
production wells into the subterranean formation, the production
well(s) including a casing cemented into place using a hydraulic
cement composition; and (c) injecting a flooding composition
including carbon dioxide and water through one or more of the
injection wells into the subterranean formation in order to
pressurize the subterranean formation and drive the hydrocarbon
toward the production well(s). The hydrocarbon and typically water
are then produced through the production well(s).
[0023] The production well(s) and injection well(s) can be placed
into the subterranean formation by drilling and completion
techniques known in the art. Typically, a plurality of injection
wells and production wells are placed in an oil field (which can
include several acres) adjacent to the subterranean formation(s) of
interest. The injection and production wells are strategically
positioned and spaced apart in the oil field to effectively and
efficiently utilize the pressure created by flooding the formation
to drive the hydrocarbon from the injection well(s) toward the
production well(s).
[0024] The hydraulic cement composition utilized in the inventive
recovery enhancing method to cement the casing into place in at
least one of the production well(s) and injection well(s) is the
inventive carbon dioxide corrosion-resistant hydraulic cement
composition. The inventive composition is preferably utilized to
cement the casing into place in all of the production wells and
injection wells utilized in the inventive recovery enhancing
method. Ideally, the inventive composition is used in connection
with all of the cementing applications carried out in association
with the inventive recovery enhancing method.
[0025] In cementing the casing into place, the inventive cement
composition is typically pumped through the tubular casing and
forced into the annular space between the outside of the casing and
the wall of the wellbore. The inventive composition then hardens
and sets to bond the casing in the wellbore and effectively seal
the casing from the formation and carbonic acid and other corrosive
fluids that may be present therein.
[0026] After the inventive cement composition is set, one or more
perforations are formed in the casing and hardened cement to allow
fluids to flow between the injection and production wells and the
formation. For example, components used to flood the formation can
be injected through perforation(s) in the injection well(s) into
the formation. Hydrocarbons, water and other fluids can be forced
from the formation through the perforation(s) into the production
well(s).
[0027] Methods of enhancing the recovery of a hydrocarbon fluid
from a subterranean formation by injecting a flooding composition
including carbon dioxide and water through one or more injection
wells into the subterranean formation in order to pressurize the
formation and drive a hydrocarbon (for example, crude oil and/or
natural gas) toward one or more production wells are well known.
The flooding composition can be injected through the injection
well(s) by alternating the injection of water and carbon dioxide
(water alternating gas (WAG) techniques) or by simultaneously
injecting water and carbon dioxide (simultaneous water and gas
injection (SWAG) techniques).
[0028] Flooding the formation with carbon dioxide and water exposes
the cement utilized to seal the casings of the production well(s)
and injection well(s) into place and in connection with other
applications associated with the wells to carbonic acid and
possibly other corrosive compounds. For example, carbonic acid,
H.sub.2CO.sub.3, readily fauns by reaction of carbon dioxide and
water. Other potentially corrosive compounds can be formed by
reactions between the carbon dioxide or carbonic acid with other
compounds in the formation.
[0029] Referring now to FIG. 1, an embodiment of the inventive
method of enhancing the recovery of a hydrocarbon fluid from a
subterranean formation is illustrated. FIG. 1 schematically
designates a subterranean formation 12 that contains a hydrocarbon
(in this case a crude oil) deposit therein.
[0030] First, an injection well 10 is placed in the subterranean
formation 12 by drilling a wellbore 16 therein. A metal tubular
casing 18 is placed into the wellbore 16 and cemented into place
therein with the inventive carbon dioxide corrosion-resistant
hydraulic cement composition 20. After the cement composition has
set, perforations 22 are formed in the casing 18 and hardened
cement composition 20 in the area of the subterranean formation 12
to allow the injection well 10 to fluidly communicate with the
formation.
[0031] A production well 30 is also placed in the subterranean
formation 12 by drilling a wellbore 32 therein. A metal tubular
casing 34 is placed into the wellbore 32 and cemented into place
therein with the inventive carbon dioxide corrosion-resistant
hydraulic cement composition 20. After the cement composition has
set, perforations 36 are formed in the casing 34 and hardened
cement composition 20 in the area of the subterranean formation 12
to allow the production well 30 to fluidly communicate with the
formation.
[0032] Next, carbon dioxide and water are injected into the
subterranean formation 12 through the injection well 10. The carbon
dioxide and water forms a flooding composition 40 in the
subterranean formation 12 that functions to pressurize the
formation and drive the hydrocarbons (crude oil in this case)
present therein toward and into the production well 30. The oil and
water are then produced from the production well 30. Due to the
fact that the cement utilized to cement the casings of the
production and injection wells into place is the inventive
composition, the cement effectively resists corrosion by the carbon
dioxide injected into the formation and related compounds formed
thereby.
[0033] Many advantages are achieved by the inventive compositions
and methods. For example, the inventive carbon dioxide-resistant
hydraulic cement composition is very effective in resisting
corrosion by high concentrations of carbon dioxide in water under
harsh temperature and pressure conditions even though it includes a
relatively low amount of Class C fly ash (when compared to certain
prior carbon dioxide-resistant hydraulic cement compositions). In
fact, in accordance with the invention, it has been discovered that
a relatively low amount of Class C fly ash (when compared to
certain prior carbon dioxide-resistant hydraulic cement
compositions) actually provides better resistance to carbonic acid
penetration into set hydraulic cement compositions. The inventive
composition is very effective in connection with the high
temperatures, high pressures and other harsh conditions that are
typically associated with downhole environments.
[0034] The present invention is exemplified by the following
example, which is given by way of example only and should not be
taken as limiting of the present invention in any way.
Example 1
[0035] The inventive carbon dioxide-resistant hydraulic cement
composition was tested in the laboratory for its ability to form
hardened hydraulic cement capable of withstanding corrosion by
carbon dioxide. Specifically, the effect of varying the amount of
the Class C fly ash utilized in the composition on the carbon
dioxide corrosion resistance of the hardened cement samples was
evaluated.
[0036] Each cement slurry was tested according to API Specification
10, Section 5. First, various hydraulic cement composition slurries
including the inventive carbon dioxide-resistant hydraulic cement
composition were prepared by admixing Class H Portland cement,
Class C fly ash (except in formulation No. 1, the control sample),
a fluid loss additive (Halad-344), a defoamer (D-Air 3000 L) and
distilled water together to form a slurry. The components and
density of the slurries are shown by Table 1 below.
TABLE-US-00001 TABLE 1 Hydraulic Cement Slurry Formulations Sample
Sample Sample Sample Formulation No. 1 No. 2 No. 3 No. 4 Portland
cement.sup.1 100% 75% 65% 50% Class C fly ash.sup.2 0 25% 35% 50%
Halad .RTM.-344.sup.3 0.25% 0.25% 0.25% 0.25% D-AIR 3000L
.TM..sup.4 0.05 g/sk 0.05 g/sk 0.05 g/sk 0.05 g/sk Water.sup.5 39%
36% 34.5% 32.6% Density.sup.6 16.4 ppg 16.4 ppg 16.4 ppg 16.4 ppg
.sup.1API Class H type Portland cement. The percent by weight is
based on the total weight of the Portland cement and Class C fly
ash (based on a dry weight basis). .sup.2Class C fly ash as defined
in ASTM Specification C 618. The percent by weight is based on the
total weight of the Portland cement and Class C fly ash (based on a
dry weight basis). .sup.3A fluid loss additive sold by Halliburton
Company and comprising a random copolymer of 2-acrylamide-2-propane
sulfonic acid and N,N-dimethyl acrylamide. The listed percent is
the percent by weight based on the total weight of the composition
(based on a dry weight basis). .sup.4A defoamer marketed by
Halliburton Company and comprising an internal olefin
(C.sub.14-C.sub.18), an alkaline hydrophobic precipitated silica,
and polypropylene glycol 4000. The recited measurement is in terms
of grams per sack of cement. .sup.5Distilled water. The listed
percent is the percent by weight based on the total weight of the
Portland cement and Class C fly ash (based on a dry weight basis).
.sup.6Pounds per gallon
[0037] Cylindrical cement core samples of each slurry formulation
(No. 1-No. 4) were then formed. Each core sample was 11/2 inches in
diameter and 21/2 long and formed by injecting the corresponding
slurry into a plastic mold and allowing the slurry to harden
therein. Each slurry was slowly poured into the mold and stirred
therein to remove any trapped air. The plastic molds were then
sealed with rubber stoppers and the samples were cured in the molds
for 15 days at a temperature of 200.degree. F. and a pressure of
2000 psi.
[0038] Following the curing period, the core samples were removed
from the molds by placing the molds in warm water to expand the
plastic and pushing the cores therefrom. Utilizing the above
procedure, two cement cores were formed for each formulation (No.
1-No. 4).
[0039] A first set of the cores (including one of each formulation
(No. 1-No. 4)) was placed into a first autoclave. A second set of
cores (including one of each formulation (No. 1-No. 4)) was placed
in the second autoclave. The samples were then carbonated for 15
days in autoclave No. 1 and 30 days in autoclave No. 2 as follows:
The chamber of each autoclave was filled with water and sealed.
Liquid carbon dioxide was then continuously injected into the water
in each chamber throughout the test periods using a sparge tube
connected to a carbon dioxide tank. The chambers of the autoclaves
were maintained at 200.degree. F. The liquid carbon dioxide was
injected into each chamber at a pressure of 2000 psi throughout the
test periods.
[0040] Following each test period (15 days for autoclave No. 1; 30
days for autoclave No. 2), the cement core samples were removed and
analyzed for carbon dioxide penetration depth therein. The depth of
carbon dioxide (carbonic acid) penetration into the core samples
was determined as follows: First, each cement core sample was cut
in half along its longitudinal axis. Each core sample was then
submerged in a 1% phenolphthalein solution. The phenolphthalein
solution turned a portion of each core sample to the color purple
which designated the portion of the sample that included calcium
hydroxide; that is, the portion that had not reacted with carbon
dioxide (carbonic acid). By measuring the thickness of the gray
portion of the sample, the carbon dioxide (carbonic acid)
penetration depth could be determined. The depth of the carbon
dioxide (carbonic acid) penetration in the core samples was
representative of the resistance (or lack of resistance) of the
core samples to corrosion by carbon dioxide (carbonic acid) under
conditions similar to the conditions encountered in downhole
environments.
[0041] The results of the tests are shown illustrated by FIG. 2 and
shown by Table 2 below.
TABLE-US-00002 TABLE 2 CO.sub.2 Penetration Depth Following 15 and
30 Days of Treatment 15 Days 30 Days Percent Class C Fly Ash.sup.1
Penetration Depth Penetration Depth 0% 4 mm 6 mm 25% 0.25 mm 0.25
mm 35% 1 mm 1 mm 50% 2 mm 3.5 mm .sup.1Class C fly ash as defined
in ASTM Specification C 618. The percent by weight is based on the
total weight of the Portland cement and Class C fly ash (based on a
dry weight basis).
[0042] The above results show that the Class C fly ash
significantly improved the resistance of the cement core samples to
penetration (and corrosion) by carbon dioxide (carbonic acid). The
results also show, surprisingly, that the degree of penetration
(and corresponding degree of corrosion) by carbon dioxide (carbonic
acid) decreased as the amount of Class C fly ash in the composition
decreased. For example, the carbon dioxide (carbonic acid)
penetration depth in the samples utilizing 25% by weight Class C
fly ash (0.25 mm) was significantly less than the carbon dioxide
(carbonic acid) penetration depth in the core samples formed using
35% by weight Class C fly ash.
[0043] Thus, the present invention is well adapted to carry out the
objects and attain the ends and advantages mentioned as well as
those which are inherent therein.
* * * * *